A light emitting diode (LED) employs a forward biased p-n junction. Vertical LEDs are a relatively new concept as are the methods for their manufacture. With vertical LEDs, the layers of the device are typically arranged in a stack, hence the term vertical. The layers of a vertical LED often include an active layer in between p-type and n-type semiconductor layers.
A common method for fabricating vertical LEDs is by removing a layer of gallium nitride (GaN) that has been grown on a sapphire substrate and bonding it to a new (thermally conductive) substrate. The GaN layers are often transferred from the growth substrate onto another substrate (e.g., molybdenum (Mo), copper-tungsten (CuW) or aluminum silicon (AlSi) wafers) by first fabricating the devices on the GaN surface, and then bonding the surface of the substrate containing the finished devices to a handle substrate using a metallic bonding process. A typical metallic bonding process used is transient liquid phase (TLP) bonding which involves relatively thick layers of precious metals (e.g., indium (In), gold (Au), and/or silver (Ag)). Thus, one drawback of this wafer bonding process is the use of precious metals which are expensive and thus raise production costs.
After depositing these metal bonding layers on the surface of the wafers, they are brought into contact (often in vacuum) with a force applying mechanism, and heated to the melting point of one of the metallic layer materials (usually In). Another drawback to this wafer bonding process is that small particles may be present on the bonding surfaces (e.g., as a result of common epitaxial growth processes used to form an LED) which will suppress bonding in that region—thereby affecting device yield.
Accordingly, improved techniques are needed for transferring GaN (or other optical device layers) from their growth substrate onto a new handle substrate.